The self-assembly of alkanethiols on gold is a well-known process that has been the subject of considerable research. (See e.g. Bain, C. D. et al., J. Am. Chem. Soc. 1989, 111, 321-335; Bain, C. D. and Whitesides, G. M. J. Am. Chem. Soc. 1988, 110. 6560-6561). The mechanisms leading to the formation of single-component and mixed self-assembled monolayers have been studied extensively. The mixed systems investigated have often consisted of methyl- and hydroxyl-terminated thiols since the results of adsorption can be readily monitored by water-contact-angle measurements. Such mixed monolayers were found to be stable and readily produced.
Chemical gradients are of great interest for numerous practical applications, such as investigating biomolecular interactions, cell-motility studies, diagnostics, nanotribology, or microfluidics, and naturally lend themselves to combinatorial studies, since an entire spectrum of chemical properties can be covered in a single experiment. A number of gradient preparation techniques for various substrates have been described (see e.g. Ruardy, T. G., et al., Surf. Sci. Rep. 1997, 29, 1-30; Liedberg, B. and Tengvall, P. Langmuir, 1995, 11, 3821-3827; Efimenko K., et al., Macromolecules 2003, 36, 2448-2453) and such gradients have been used for further experiments and applications (Herbert C. B., at al., Chem. Biol. 1997, 4, 731-737; Sehayek T., Vaskevich A. and Rubinstein I. J. Am. Chem. Soc. 2003, 125, 4718-4719). Several methods have been reported for the generation of thiol-based chemical gradients, including (1) the cross-diffusion of two thiol solutions through a polysaccharide matrix (Liedberg, B. and Tengvall, P. Langmuir, 1995, 11, 3821-3827), (2) applying an electrochemical potential to a substrate during adsorption (Terrill R. H., et al., J. Am. Chem. Soc 2000, 122, 988-989), (3) the use of microfluidic devices (Jeon N. L., et al., Langmuir 2000, 16, 8311-8316; Dertinger S. K. W., et al., Anal. Chem. 2001, 73, 1240-1246), and (4) scanning-tunneling-microscopy-based replacement lithography (Fuierer R. R., et al., Adv. Mater. 2002, 14, 154-157). With the exception of the electrochemical potential approach, the gradients formed have been limited in physical size. The electrochemical approach is a complicated process that is limited to forming gradients on conducting substrates.
Therefore it is an object of the invention to find a simpler method for fabricating chemical gradients of adsorbate monolayers on a variety of substrates.
It is also an object of the invention to provide a method for fabricating chemical gradients of adsorbate monolayers that are of the order of one cm or longer.